Studies on the growth of reticulum cell sarcoma (RCS) in SJL mice have been extended to include growth in F1 hybrids. Several interesting effects of the H-2-region on RCS growth and on the ability of lymphoid cell of F1 mice to give proliferative responses to RCS were identified and are being examined further. Further studies on the effect of T and B cells in the host on growth of RCS will be analyzed. The tumor cells as well as the host cells rresponding to the RCS make type II interferon. This may provide an explantaion for the immuno-suppresive properties of RCS, but the tumor cells also cause production of an immuno-stimulatory factor which needs further study. Growth of PU-5 in F1 mice of BALB/c will also be studied in more detail, as will the nature of the cell type which promotes growth of PU-5 in vivo and in vitro.